There is a large body of data on the properties of human transferrin but major questions of iron metabolism in health and disease remain. We have designed a rat model that can be used to clarify the mechanisms of iron transfer, uptake and utilization. The invasive techniques required to answer these questions are not applicable to human studies. In our rat model, however, we use such techniques to yield data on iron transfer that can be applied to the clarification of human disorders of iron metabolism. All mammalian cells require iron which is delivered to the cell by the plasma iron transport protein, transferrin. The first step in this transfer is interaction between transferrin and cell membrane. The mechanism of this interaction is unknown with respect to either the surrender or the binding of transferrin iron. Questions still unanswered relate to the functional role of transferrin during absorption, catabolism, storage and utilization of iron. We propose to identify the unique rat transferring iron-binding site which donates its iron to the reticulocyte by virtue of its pH-dependent iron dissociation properties. Once established, this site will be used as a reference site to identify the specific iron-binding site involved in exchange of iron in other tissues. Rat transferrin iron-binding and iron-donating properties will be studied in order to determine: a) whether each iron-binding site is equivalent; b) the function of each site during iron delivery and acquisition; and c) whether the relative concentration of diferric transferrin and monoferric transferrin determines when iron is deposited or removed from storage. The investigation of transferrin's pH dependent iron dissociation properties can help to develop methods of assaying human plasma for monoferric and diferric transferrin content and thus improve diagnostic and possible therapeutic capabilities in human diseases of iron deficiency and iron overload.